HVAC Geekery

Today we are having record-breaking heat in the Portland/Vancouver area, and so my A/C unit decided now would be a good time to stop working. I checked various things like:
1. Is the thermostat set correctly?
2. Are there batteries in the thermostat that need to be changed
3. Are any breakers tripped?
4. Is the heat pump fan spinning?

The answer to that last one was No. The air-handler/furnace was blowing all of the time, but the heat pump outside was just sitting there idle. Every now and then I could hear a high voltage hum from it. The fan motor was hot, like it was trying to spin, but wasn’t.

I described the symptoms to my electrician and HVAC (Heating, Ventilation, Air Conditioning) techs at work and they asked “Will the fan spin up if you stick a dowel in and spin it?” I went back and found that yes, it would! I called them and they said in unison “Bad capacitor.” They each had this very same issue last month. In the heat pump is a large metal can capacitor and a big relay called a “connector”. If the capacitor goes bad, the fan will still be able to spin, but the compressor that makes cold won’t work.

They gave me the address of a local vendor and I bought a replacement capacitor for $8. The numbers to match are the microfarads and voltage. It can be rated higher, but not lower. Best just to shoot for a match. Also you need to know if it is a dual capacitor with 3 terminals or not. Mine was.

NOTE: The next steps involve high-voltage (240VAC) and should not be performed unless you are comfortable and knowledgeable about handling such energy, have a volt meter, and the right tools for the job . When in doubt, hire an expert. Mine was a 45uf+5uf, 370VAC. I found this information by looking inside the back cover of the heat pump. I turned off the breaker that said it went to the A/C unit. I also removed the large fuse set on the wall right next to the heat pump. This ensures that no electricity will be going to the parts I’ll be working around. I removed the back cover of the heat pump and there was the large, hand-sized capacitor and a large relay.

I took photos of the old capacitor to show clearly where the colored wires connected. replaced the capacitor, but had to make the previous bracket fit since the new capacitor was larger diameter. Somehow during the whole process I managed to cause a spark on the “connector” relay. I didn’t think much of it until everything was installed, but not working. The thermostat was blank, meaning no power to it. It turns out that I blew a fuse on the furnace circuit board that is controlled by the thermostat and which controls the “connector”. I spoke with my work techs and they said to look for a small fuse, which turned out to be an automotive blade-type fuse rated for 3 amps. I went down to an auto parts store and bought a few.

Once that was installed, I replaced the main fuse to the heat pump and powered on the breaker. The air-handler fan turned on and the thermostat turned on, and then the heat pump began working! I learned quite a bit from this project, and likely saved hundreds in a service call. Keep in mind, I did have experts right there (well, over the phone) to guide me. Don’t try this unless you have such guidance. It is really easy to throw the wrong breaker and think things are powered down and get a bad shock or worse.




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